1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, printer, or the like and, more particularly, to an electrophotography type or electrostatic recording type image forming apparatus.
2. Description of the Related Art
Conventionally, electrophotography type or electrostatic recording type image forming apparatuses for forming an electrostatic latent image on an image carrier, and developing the electrostatic latent image with a toner have been popularly used.
In recent years, apparatuses for forming full-color images have also been popularly used.
FIG. 7 shows an example of an electrophotography type full-color image forming apparatus.
In a color image forming apparatus shown in FIG. 7, a photosensitive drum 1 as an image carrier is supported to be rotatable in the arrow in FIG. 7 arrow in FIG. 7. A pre-exposure lamp 11, a corona (primary) charger 2, an optical system 3, a developing device 4, a transfer device 5, and a cleaning device 6 are arranged around the photosensitive drum 1.
The optical system 3 comprises, e.g., a laser beam exposure device which is constituted by an original such as a document scanning unit and color-separation filters, and irradiates color-separated optical images or a corresponding optical image E onto the photosensitive drum 1.
An optical image E is irradiated onto the photosensitive drum 1, which is charge-removed by the pre-exposure lamp 11 and is then uniformly charged by the primary charger 2, in units of separated colors, thereby forming a latent image. The developing device 4 comprises rotary of developers. More specifically, four developers, i.e., a black developer 4K, a cyan developer 4C, a magenta developer 4M, and a yellow developer 4Y are arranged on a rotary member 4a, which rotates about a central shaft 4b. A predetermined developer is rotated to a developing position opposing the photosensitive drum 1 to develop the latent image on the photosensitive drum 1, thereby forming a toner image on the photosensitive drum 1 using a resin-based toner.
Furthermore, the toner image on the photosensitive drum 1 is transferred onto a recording medium which is supplied from a recording medium cassette 7 (one of cassettes 7a, 7b, and 7c) to a position opposing the photosensitive drum 1 via a convey system and the transfer device 5 along a paper path indicated by a dotted line in FIG. 7.
In this example, the transfer device 5 comprises a transfer drum 5a, a transfer corona charger 5b, an attraction roller 5g opposing an attraction corona charger 5c for electrostatically attracting a recording medium, an inner corona charger 5d, and an outer corona charger 5e. A cylindrical recording medium carrier sheet 5f consisting of a dielectric integrally extends on a circumferential aperture region of the transfer drum 5, which is rotatably and axially supported.
As the transfer drum 5a rotates, the toner image on the photosensitive drum 1 is sequentially transferred onto a recording medium carried on the recording medium carrier sheet 5f by the transfer charger 5b. A desired number of color images are transferred onto the recording medium attracted on the recording medium carrier sheet 5f, thus forming a full-color image.
Upon completion of the desired number of toner images, the recording medium is peeled from the transfer drum 5a by a peeling means 8, and is exhausted onto a tray 10 via a thermal roller fixing device 9.
On the other hand, after the transfer operation, the residual toner on the surface of the photosensitive drum 1 is cleaned by the cleaning device 6, and thereafter, the photosensitive drum 1 is subjected to image forming processes again. The recording medium carrier sheet 5f extending on the transfer drum 5a is charge-removed by the inner and outer corona chargers 5d and 5e.
FIG. 8 is a timing chart showing the operation timings of the respective units in image formation in the image forming apparatus described above. An actual image forming operation is started after an ITOP signal for determining the position of the transfer drum 5a is detected.
During rotation of the photosensitive drum prior to the image formation starting from laser exposure, the photosensitive drum is subjected to pre-exposure and primary charging over one revolution to stabilize the potential (the rotation of the image carrier which is performed before image formation for the purpose of stabilizing the charged potential by charge removal and charging over one revolution will be referred to as the rotation for stabilizing the potential hereinafter).
However, when a large-diameter photosensitive drum which has a circumferential length ratio of 1:1 with respect to the transfer drum is used, or when the apparatus is rendered compact, the convey time of a recording medium from the paper feed cassette to the transfer device may become shorter than the time required for one revolution of the photosensitive drum.
In this case, the photosensitive drum must be rotated by one extra revolution to attain the above-mentioned rotation for stabilizing the potential, and the first image forming time (to be referred to as "FCOT" hereinafter) after the rest time is undesirably prolonged.